Timepieces having a calendar display typically use an annular rotating wheel (such as a date wheel or day wheel) to display calendar information. The numbers 1 to 31 are printed around the outside of this rotating wheel to display the date, and letters such as Sun to Sat are printed to display the day. This rotating wheel is driven to sequentially present the calendar information through a window in the dial.
Display mechanisms for retrograde timepieces that have date, day, or time scales printed in fan-shaped display areas on the face of the dial and use hands to indicate the date and time are also known. To display the weekday, for example, on such a retrograde timepiece, a hand sequentially points to Sun, Mon, Tue, . . . Sat, and then returns to Sun. To achieve this flyback hand action in such a timepiece, Japanese Unexamined Patent Appl. Pub. H11-6880A1 (claim 2, paragraphs [0002] and [0016], and FIG. 3) teaches rendering a spiral thread on the outside circumference of a conventional ring-shaped date wheel (except that the dates are not printed on the wheel) disposed behind the main plate and using this as a cam to display the date. More specifically, Japanese Unexamined Patent Appl. Pub. H11-6880A1 teaches rotatably rendering a pinion to which the hand is attached and a pair of racks that mesh with this pinion from mutually opposite directions on the main plate. Spring pressure exerted through one rack causes the other rack to contact the outside of the rotating cam wheel, thus causing the hand to move reciprocally according to rotation of the date wheel.
The timepiece taught in Japanese Patent 3140700B2 (FIG. 1 and FIG. 6) has a day-turning wheel to which drive force from a spring is transferred, a day wheel that is driven by the day-turning wheel, a cam fixed on this day wheel, a lever that is contacted by this cam and moves circularly, a small day wheel that is driven by a rack formed on this lever, and a small day wheel spring in which power is stored in conjunction with rotation of the small day wheel. Rotation of the lever in contact with the cam thus causes the small day wheel to turn and the hand to move reciprocally.